My theory is not that literal. Posterity of We the People does not mean the direct descendants of the original citizens.
"We the People" is a term of art to represent the citizens at large - at any time. The Posterity of We the People means the children of citizens. A foreign-born naturalized citizen becomes one of the body of We the People, and then their future children (their posterity) are natural born citizens.
-PJ
They didn’t mean it literally either. “Descendants” does not imply only direct descendants.